Should we really document specific USE_EXPAND variables? What about all the others?
USE_EXPAND="ABI_MIPS ABI_X86 ALSA_CARDS APACHE2_MODULES APACHE2_MPMS CALLIGRA_FEATURES CAMERAS COLLECTD_PLUGINS CROSSCOMPILE_OPTS CURL_SSL DRACUT_MODULES DVB_CARDS ELIBC ENLIGHTENMENT_MODULES FCDSL_CARDS FFTOOLS FOO2ZJS_DEVICES FRITZCAPI_CARDS GPSD_PROTOCOLS GRUB_PLATFORMS INPUT_DEVICES KERNEL LCD_DEVICES LIBREOFFICE_EXTENSIONS LINGUAS LIRC_DEVICES MONKEYD_PLUGINS NETBEANS_MODULES NGINX_MODULES_HTTP NGINX_MODULES_MAIL OFED_DRIVERS OFFICE_IMPLEMENTATION OPENMPI_FABRICS OPENMPI_OFED_FEATURES OPENMPI_RM PHP_TARGETS PYTHON_SINGLE_TARGET PYTHON_TARGETS QEMU_SOFTMMU_TARGETS QEMU_USER_TARGETS RUBY_TARGETS SANE_BACKENDS USERLAND VIDEO_CARDS VOICEMAIL_STORAGE XFCE_PLUGINS XTABLES_ADDONS
